What, exactly, are you looking for HMVC to give you? I'd be willing to guess that most of it already exists within CI4.

A controller is just a class, so you can always create a new instance of any controller (passing the required classes along) and call it, which seems to be one of the hallmarks of HMVC. It is safer to do that now then it was with the hacks necessary with any of the solutions in CI3.

Namespaces solve 80%+ of the organizational aspects that HMVC provides, which is one of the 2 primary things I've seen people use it for.

The remaining 20% is already handled by allowing language files, helpers, views, etc, to be scanned for, allowing fully modular code if you choose to use it.

The final thing I've seen people use it for was the Modules::run() that Wiredesignz's solution provided. View Cells handles that feature.